In the production of building boards frequently a spreading process, i.e. a dry or quasi-dry manufacturing process, is used. A mixture of binders, reinforcement materials and fillers is spread on a moving forming belt and subsequently densified in a press. As is for instance known from DE-OS 34 39 493, the layer can be built up by several layers spread onto the forming conveyor belt.
In the DE-OS 37 19 129 a process for the continuous formation of a uniform layer of loose material as well as an installation for carrying out the process are described. The continuously incoming loose material is deposited in strips by means of a gravity chute oscillating across the receiving belt. The deposition period and the speed of the receiving belt are attuned to each other, so that one strip comes to lie next to the other, forming a gap-free coating of the receiving belt. In addition to this adjustment, the translatory speed of the gravity chute can be finely varied, in order to systematically even out the height variations of the deposited layer occurring over the width of the spread layer.
For this purpose the thickness of the spread layer is measured and the results are used for setting the speed profile of the oscillating gravity chute. In this way the naturally established mass profile can be counteracted.
In a further development a stripper roll designed as a spiked roller is arranged above the receiving belt, and driven in a direction of rotation selected so that it throws the projecting material in the travel direction of the receiving belt. This step is particularly important when quick-setting, wetted, gypsum-containing masses are supposed to be spread.
In this process height variations of the plate of 5 to 10% still occur. Such height variations occur especially in wider plates with a width of approximately 2.50 m. For the correspondingly larger widths, the height of the gravity chute is also increased as necessary. Thereby the stream of falling loose material reaches a considerable speed and kinetic energy, which result in uncontrollable movements of the spread material upon impact on the receiving belt. In this way a gap-free covering of the belt is not insured. A rather wavy layer which can even have holes is formed.
Under these conditions the spiked rollers are not in a position to level the irregularities, when they work in the travel direction. In practical applications it has been found that a uniformization is possible only in the conventional manner, when the spiked rollers work in the opposite direction and strip back the projecting material.
This procedure has considerable disadvantages when wet, gypsum-containing mixtures are spread. The reason is that behind the roller accumulations of material are formed which rotate, and in the core of which material can be caught for periods of minutes.